Filtration systems of the kind referred to are generally comprised of a number of elongated, generally rectangular, filter cells or beds spaced in side-by-side relationship. Each cell contains a single granular filter media, such as sand, or two kinds of media, such as sand and pulverized anthracite coal. Water to be filtered is delivered from above to flood the cells, the water flowing downwardly through the media by gravity leaving particulate matter entrapped in the spaces between the media particles. The bed of filter material within each cell rests on a support plate commonly called a drainplate which in the past was formed of a porous ceramic material. More recently, porous drainplates made from thermoplastic materials have been substituted for the drainplates formed of ceramic materials. Amongst the advantages of such plastic plates are substantially reduced weight and material costs, lower labor and shipping costs, and simplified installation. The plates are tough and resilient and can be easily joined together. They can be made longer than the ceramic plates thereby minimizing joints. They can be easily hand-sawed for custom field fits where required.
In filtering equipment of the kind described, a traveling backwash hood mounted over the cells moves from cell to cell and is used in conjunction with a pump to periodically backwash each cell and remove particulate and other debris from the drainplate and the filtration media. An advantage of this equipment is that while one cell is shut down, others are in operation, thereby providing substantially continuous filtration.
A problem with thermoplastic drainplates of the kind referred to is that they tend to bow or bend in planes extending transversely of their length under the combined loading of the filter media and the water. When overloaded, the sides of the plates pull away from the vertical sidewalls of the cells. This results in a channelling of the water down the sides of the cells around the sides of the drainplates thereby reducing the efficiency of the filter and of the backflushing operations.